Multicast client: add bi-directional support to receive Pub/Sub and publish multicast

The multicast publisher for Datashare currently joins a multicast group and tunnels packets arriving on the specified group over a Pub/Sub topic that is also specified.

This request is to add a feature where the traffic flow can be reversed, the client is passed a topic name or a subscription as a parameter and receives topic messages. In reverse mode, the multicast client will take the messages from Pub/Sub and send them outbound over the Multicast interface.